Injection nozzle systems for hot runner injection systems are well known and include systems such as the SDVG 750 single drop valve gate, SDHT 750 single drop hot sprue and ULTRA systems sold by the assignee of the present invention. Such injection nozzle systems are constructed to very close tolerances to accommodate the high pressures typically employed in hot runner injection molding and to provide the necessary degree of process control to reduce visible gate vestiges on the molded items, etc.
In particular, avoiding the formation of visible gate vestiges is very much desired within the industry. As is known to those of skill in the art, the formation of visible gate vestiges is mitigated by good process control, the high tolerances used in the construction of the nozzle system and by good thermal properties in the nozzle. In particular, to obtain acceptable thermal properties, it is desired that the heater elements in the nozzle system be located as closely as possible to the nozzle tip to ensure rapid and efficient heating of resin in the gate area. Of course, the mold plates must also be able to provide sufficient cooling to the gate to freeze the resin when the injection portion of the molding cycle is complete.
Reference is made in this regard to U.S. Pat. No. 5,046,942 to Gellert which teaches a thermal-gated nozzle wherein a second heating element is located adjacent the mold gate area and embedded inside a two part nozzle tip to provide the desired heating of the nozzle tip.
One problem which occurs with some prior nozzle systems, such as that taught in Gellert, is that a mechanical contact is present between the nozzle tip and the mold gate area that allows thermal transfer to occur between these two members. Accordingly, undesirable cooling of the nozzle tip occurs, as does undesirable heating of the gate by the heater elements on the nozzle and/or nozzle tip.
Attempts have been made in the past to mitigate these problems by providing insulation between the gate and the nozzle. For example, U.S. Pat. No. 5,208,052 to Schmidt et al., assigned to the assignee of the present invention, teaches an injection nozzle with a removable nozzle tip made of material with a relatively high thermal conductivity which is surrounded by a removable thermal insulation sleeve made of material with a lower thermal conductivity, which also acts as a seal to intrusion of resin into unwanted regions of the nozzle system. A similar concept is also described in U.S. Pat. No. 5,299,928 to Gellert, wherein the insulation sleeve is placed between the nozzle tip and the housing of the injection nozzle. More recently, U.S. Pat. No. 5,569,475 to Adas et al. discloses several types of insulation coating layers that are used in hot runner systems adjacent the mold gate area.
However, each of these prior systems, and all others of which the present inventor is aware, suffer from one or more disadvantages and/or drawbacks. For example, the approach shown in Adas has very limited application since the insulation layers taught are very thin and provide little impediment to unwanted heat transfer. Also the layers taught in Adas are not very strong mechanically and are thus susceptible to mechanical failure. While the insulation sleeve disclosed by Schmidt in U.S. Pat. No. 5,208,052 and the similar sleeve disclosed by Gellert in U.S. Pat. No. 5,299,928 are reasonable insulators and are reasonably robust mechanically, these nozzle systems are generally not satisfactory for molding articles from abrasive resins, such as the so-called engineering grade resins of which NORYL GTX is an example, as such resins damage the mold gate through abrasive wear requiring replacement of the entire mold plate in which the mold gate is formed. Further, these nozzle systems are generally not generally satisfactory for molding articles from heatsensitive materials, such as crystalline resins of which PA 66 is an example, as such materials require more heat at the gate area than other resins. As the mold gate is embedded in the mold plate in these prior systems and is thus subject to heat loss to the mold plate which is cooled by the mold cooling system, poor thermal performance is obtained and the required heat at the gate cannot reliably be obtained.
It is desired to have a injection nozzle system with good thermal properties and which provides enhanced serviceability and relatively easy assembly and disassembly.